1'virt' Generic Virtual Platform (``virt``) 2========================================== 3 4The ``virt`` board is a platform which does not correspond to any real hardware; 5it is designed for use in virtual machines. It is the recommended board type 6if you simply want to run a guest such as Linux and do not care about 7reproducing the idiosyncrasies and limitations of a particular bit of 8real-world hardware. 9 10Supported devices 11----------------- 12 13The ``virt`` machine supports the following devices: 14 15* Up to 8 generic RV32GC/RV64GC cores, with optional extensions 16* Core Local Interruptor (CLINT) 17* Platform-Level Interrupt Controller (PLIC) 18* CFI parallel NOR flash memory 19* 1 NS16550 compatible UART 20* 1 Google Goldfish RTC 21* 1 SiFive Test device 22* 8 virtio-mmio transport devices 23* 1 generic PCIe host bridge 24* The fw_cfg device that allows a guest to obtain data from QEMU 25 26The hypervisor extension has been enabled for the default CPU, so virtual 27machines with hypervisor extension can simply be used without explicitly 28declaring. 29 30Hardware configuration information 31---------------------------------- 32 33The ``virt`` machine automatically generates a device tree blob ("dtb") 34which it passes to the guest, if there is no ``-dtb`` option. This provides 35information about the addresses, interrupt lines and other configuration of 36the various devices in the system. Guest software should discover the devices 37that are present in the generated DTB. 38 39If users want to provide their own DTB, they can use the ``-dtb`` option. 40These DTBs should have the following requirements: 41 42* The number of subnodes of the /cpus node should match QEMU's ``-smp`` option 43* The /memory reg size should match QEMU’s selected ram_size via ``-m`` 44* Should contain a node for the CLINT device with a compatible string 45 "riscv,clint0" if using with OpenSBI BIOS images 46 47Boot options 48------------ 49 50The ``virt`` machine can start using the standard -kernel functionality 51for loading a Linux kernel, a VxWorks kernel, an S-mode U-Boot bootloader 52with the default OpenSBI firmware image as the -bios. It also supports 53the recommended RISC-V bootflow: U-Boot SPL (M-mode) loads OpenSBI fw_dynamic 54firmware and U-Boot proper (S-mode), using the standard -bios functionality. 55 56Machine-specific options 57------------------------ 58 59The following machine-specific options are supported: 60 61- aclint=[on|off] 62 63 When this option is "on", ACLINT devices will be emulated instead of 64 SiFive CLINT. When not specified, this option is assumed to be "off". 65 66Running Linux kernel 67-------------------- 68 69Linux mainline v5.12 release is tested at the time of writing. To build a 70Linux mainline kernel that can be booted by the ``virt`` machine in 7164-bit mode, simply configure the kernel using the defconfig configuration: 72 73.. code-block:: bash 74 75 $ export ARCH=riscv 76 $ export CROSS_COMPILE=riscv64-linux- 77 $ make defconfig 78 $ make 79 80To boot the newly built Linux kernel in QEMU with the ``virt`` machine: 81 82.. code-block:: bash 83 84 $ qemu-system-riscv64 -M virt -smp 4 -m 2G \ 85 -display none -serial stdio \ 86 -kernel arch/riscv/boot/Image \ 87 -initrd /path/to/rootfs.cpio \ 88 -append "root=/dev/ram" 89 90To build a Linux mainline kernel that can be booted by the ``virt`` machine 91in 32-bit mode, use the rv32_defconfig configuration. A patch is required to 92fix the 32-bit boot issue for Linux kernel v5.12. 93 94.. code-block:: bash 95 96 $ export ARCH=riscv 97 $ export CROSS_COMPILE=riscv64-linux- 98 $ curl https://patchwork.kernel.org/project/linux-riscv/patch/20210627135117.28641-1-bmeng.cn@gmail.com/mbox/ > riscv.patch 99 $ git am riscv.patch 100 $ make rv32_defconfig 101 $ make 102 103Replace ``qemu-system-riscv64`` with ``qemu-system-riscv32`` in the command 104line above to boot the 32-bit Linux kernel. A rootfs image containing 32-bit 105applications shall be used in order for kernel to boot to user space. 106 107Running U-Boot 108-------------- 109 110U-Boot mainline v2021.04 release is tested at the time of writing. To build an 111S-mode U-Boot bootloader that can be booted by the ``virt`` machine, use 112the qemu-riscv64_smode_defconfig with similar commands as described above for Linux: 113 114.. code-block:: bash 115 116 $ export CROSS_COMPILE=riscv64-linux- 117 $ make qemu-riscv64_smode_defconfig 118 119Boot the 64-bit U-Boot S-mode image directly: 120 121.. code-block:: bash 122 123 $ qemu-system-riscv64 -M virt -smp 4 -m 2G \ 124 -display none -serial stdio \ 125 -kernel /path/to/u-boot.bin 126 127To test booting U-Boot SPL which in M-mode, which in turn loads a FIT image 128that bundles OpenSBI fw_dynamic firmware and U-Boot proper (S-mode) together, 129build the U-Boot images using riscv64_spl_defconfig: 130 131.. code-block:: bash 132 133 $ export CROSS_COMPILE=riscv64-linux- 134 $ export OPENSBI=/path/to/opensbi-riscv64-generic-fw_dynamic.bin 135 $ make qemu-riscv64_spl_defconfig 136 137The minimal QEMU commands to run U-Boot SPL are: 138 139.. code-block:: bash 140 141 $ qemu-system-riscv64 -M virt -smp 4 -m 2G \ 142 -display none -serial stdio \ 143 -bios /path/to/u-boot-spl \ 144 -device loader,file=/path/to/u-boot.itb,addr=0x80200000 145 146To test 32-bit U-Boot images, switch to use qemu-riscv32_smode_defconfig and 147riscv32_spl_defconfig builds, and replace ``qemu-system-riscv64`` with 148``qemu-system-riscv32`` in the command lines above to boot the 32-bit U-Boot. 149